A Coupled EXAFS-Molecular Dynamics Study on PuO 2 + and NpO 2 + Hydration: The Importance of Electron Correlation in Force-Field Building.

The physicochemical properties of the monovalent actinyl cations, PuO 2 + and NpO 2 + , in water have been studied by means of classical molecular dynamic simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The TIP4P water model was adopted. Given the paramagnetic character of these actinyls, the cation-water interaction energies were computed from highly correlated wave functions using the NEVPT2 method. It is shown that the multideterminantal character of the wave function has a relevant effect on the main distances of the hydrated molecular cations. Several structural, dynamical, and energetic properties of the aqueous solutions have been obtained and analyzed. Structural RDF analysis gives An-O yl distances of 1.82 and 1.84 Å and An-O(water) distances of 2.51 and 2.53 Å for PuO 2 + and NpO 2 + in water, respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO 2 + and NpO 2 + are revisited and analyzed, assuming tetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MD simulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that both PuO 2 + and NpO 2 + cations in water are stable pentahydrated aqua ions.